1. Field of the Invention
The present invention relates generally to the field of image processing. More particularly, the present invention relates to the scaling of image streams that use motion vectors.
2. Related Art
In response to more technically demanding audio-video consumers, suppliers of audio-video components have provided high-definition television, video-on-demand and set-top devices. To allow for cross-compatibility between newly developed systems, a panel of experts has developed an international standard for generic coding of moving pictures and associated audio information. This standard, designated MPEG (see the Motion Picture Experts Group-2 Specification ISO/IEC JTC1/SC29/WG11 N0702, May 10, 1994), includes standards for video image compression and decompression.
Video image compression takes advantage of redundant information inherent in moving pictures. With the exception of scene changes, successive frames of a moving picture do not change substantially in form or content. This is because there is typically little change of information from one frame to the next. Thus, there is no need to directly encode all image components (pixels, YUV format, etc.) of each frame for transmission purposes. The bandwidth saved by not sending all information for each frame may be used to send a different frame.
Picture-in-Picture (PIP) displays a reduced size image in a display of a full-size image. A reduced-sized image is called a scaled image, and the process for reducing the size of the image is called scaling. Typically, scaled images are not transmitted by service providers, such as local cable television companies. Therefore, a device located at the consumer's location must perform any required scaling.
It is desirable to be able to process and present reduced-size video streams for applications such as PIP in set-top decoder products, and the like. However, due to limited resources and cost constraints, set-top decoder products rarely have sufficient processing power or memory resources to decode multiple full-size images simultaneously. Thus, it is not desirable to decode a full stream, and scale it after all decoding is complete.
Methods for image scaling have been proposed by Dr. Chang et al., in an article titled "Manipulation and Compositing of MC-DCT Compressed Video," (IEEE Journal on Selected Areas of Communications, Vol. 13, No. 1, January, 1995). Chang et al.'s method operates entirely in the discrete cosine transform (DCT) domain, which complicates its implementation from memory and computation perspectives. Unfortunately, Chang et al.'s method is subject to variable throughput problems, and applies additional DCTs for motion compensation scaling. Furthermore, Chang et al.'s method, because it is done exclusively in the frequency domain, is subject to undesirable visual artifacts which are not easily removed.
What is needed is a system for scaling video streams in the spatial domain that is applicable to a system with limited resources, such as set-top boxes, while avoiding unnecessary consumption of transmission bandwidth between the service provider and the system.